Tissier's knee was injured some years ago. Someone was thrown full force into his 's knee from the side. He managed to get back on the tatami, but never recoverd from that incident.
Watching him, you may sometimes recognize that his steps are not allways so precise like they used to be. And sometimes he is equilibrium is not as good as it could or should be.
But this is not the reason for the way he shows aikidō that is discussed here. It's a problem apart from his teaching method or understanding of demonstration.

It's sad that his knee still inhibits him so much.

I wasn't making the same criticism as the others. The main thing that stood out to me was his obvious discomfort and inability to bend that knee. Apart from that, I am a fan of Tissier.